Sequential beam splitting in a radiation sensing apparatus

ABSTRACT

Systems, methods, and apparatuses for providing electromagnetic radiation sensing using sequential beam splitting. The apparatuses can include a micro-mirror chip having a plurality of light reflecting surfaces, an image sensor having an imaging surface, and a beamsplitter unit located between the micro-mirror chip and the image sensor. The beamsplitter unit includes a plurality of beamsplitters aligned along a horizontal axis that is parallel to the micro-mirror chip and the imaging surface. The beamsplitters implement the sequential beam splitting. Because of the structure of the beamsplitter unit, the height of the arrangement of the micro-mirror chip, the beamsplitter unit, and the image sensor is reduced such that the arrangement can fit within a mobile device. Within a mobile device, the apparatuses can be utilized for human detection, fire detection, gas detection, temperature measurements, environmental monitoring, energy saving, behavior analysis, surveillance, information gathering and for human-machine interfaces.

RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 17/066,396, filed Oct. 8, 2020, which is acontinuation application of U.S. patent application Ser. No. 16/400,831,filed May 1, 2019, issued as U.S. Pat. No. 10,801,896 on Oct. 13, 2020,which claims priority to Prov. U.S. Pat. App. Ser. No. 62/791,193, filedJan. 11, 2019, entitled “ON-BOARD RADIATION SENSING APPARATUS”, Prov.U.S. Pat. App. Ser. No. 62/791,195, filed Jan. 11, 2019, entitled“SEQUENTIAL BEAM SPLITTING IN A RADIATION SENSING APPARATUS”, and Prov.U.S. Pat. App. Ser. No. 62/791,479, filed Jan. 11, 2019, entitled“RADIATION SENSING APPARATUS WITH A LIGHT SOURCE MOUNTED ON A FLEXIBLEPART”, the entire disclosures of which applications are herebyincorporated herein by reference.

FIELD OF THE TECHNOLOGY

At least some embodiments disclosed herein relate to electromagneticradiation detection using sequential beam splitting in general and moreparticularly but not limited to the sensing of infrared (IR) radiationusing sequential beam splitting in a radiation sensing apparatus.

And, at least some embodiments disclosed herein relate toelectromagnetic radiation detection using beam splitting in general andmore particularly but not limited to the sensing of infrared (IR)radiation using beam splitting in a radiation sensing apparatus with thelight source mounted on a flexible part. Also, disclosed herein is aprinted circuit board arrangement with a flexible part for anelectromagnetic radiation detector.

BACKGROUND

U.S. Pat. No. 9,857,229 discloses a method of fabricatingelectromagnetic radiation detection devices including: forming a firstmask on a substrate; forming a structural layer on the substrate usingthe first mask; forming a metallic layer overlying the structural layer;removing the first mask; forming a second mask on the substrate, thesecond mask having mask openings; selectively patterning the metalliclayer using the mask openings; and removing the second mask. The entiredisclosure of U.S. Pat. No. 9,857,229 is hereby incorporated herein byreference.

U.S. Pat. No. 5,929,440 discloses an electromagnetic radiation detectorthat has an array of multi-layered cantilevers. Each of the cantileversis configured to absorb electromagnetic radiation to generate heat andthus bend under the heat proportionately to the amount of absorbedelectromagnetic radiation. The cantilevers are illuminated and lightreflected by the bent cantilevers are sensed to determine the amount ofelectromagnetic radiation. The entire disclosure of U.S. Pat. No.5,929,440 is hereby incorporated herein by reference.

U.S. Pat. No. 9,851,256 discloses a radiation detection sensor includinga plurality of micromechanical radiation sensing pixels having areflecting top surface and configured to deflect light incident on thereflective surface as a function of an intensity of sensed radiation.The sensor can provide adjustable sensitivity and measurement range. Theentire disclosure of U.S. Pat. No. 9,851,256 is hereby incorporatedherein by reference.

U.S. Pat. No. 9,810,581 discloses an electromagnetic radiation sensingmicromechanical device to be utilized in high pixel-density pixel sensorarrays. Arrays of the device can be utilized as IR imaging detectors.The entire disclosure of U.S. Pat. No. 9,810,581 is hereby incorporatedherein by reference.

SUMMARY OF THE DESCRIPTION

Described herein are systems, methods, and apparatuses for providingelectromagnetic radiation sensing using sequential beam splitting. Theapparatuses can include a micro-mirror chip having a plurality of lightreflecting surfaces, an image sensor having an imaging surface, and abeamsplitter unit located between the micro-mirror chip and the imagesensor. The beamsplitter unit includes a plurality of beamsplittersaligned along a horizontal axis that is parallel to the micro-mirrorchip and the imaging surface. The beamsplitters implement the sequentialbeam splitting. Because of the structure of the beamsplitter unit, theheight of the arrangement of the micro-mirror chip, the beamsplitterunit, and the image sensor is reduced such that the arrangement can fitin a space constrained device, such as a mobile device. Configuredwithin a device, the apparatuses can be utilized for human detection,fire detection, gas detection, temperature measurements, environmentalmonitoring, energy saving, behavior analysis, surveillance, informationgathering and for human-machine interfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be understood more fully from the detaileddescription given below and from the accompanying drawings of variousembodiments of the disclosure.

FIG. 1 illustrates an apparatus 100 configured to measure a distributionof electromagnetic radiation according to at least one embodiment.

FIG. 2 illustrates some parts of the apparatus illustrated in FIG. 1 ,and further illustrates the micro-mirror chip of the apparatus.

FIG. 3 illustrates some parts of the apparatus illustrated in FIG. 1 ,and further illustrates the beamsplitter unit of the apparatus.

FIG. 4 illustrates some parts of the apparatus illustrated in FIG. 1 ,and further illustrates mechanisms for displacements of reflected lightrays on the imaging surface of the apparatus to determine the intensityof electromagnetic radiation on micro mirrors of the apparatus.

FIG. 5 illustrates another apparatus 500 configured to measure adistribution of electromagnetic radiation according to at least oneother embodiment where the light source is part of the printed circuitboard.

FIG. 6 illustrates another apparatus 600 configured to measure adistribution of electromagnetic radiation according to at least oneother embodiment where a signal processing unit is integrated with ordirectly attached to the image sensor.

FIGS. 7A, 7B, and 7C illustrate a construction for the structure of abeamsplitter unit according to at least one embodiment.

FIG. 8 illustrates another apparatus 800 configured to measure adistribution of electromagnetic radiation according to at least oneother embodiment where the apparatus includes a light source mounted ona flexible part.

DETAILED DESCRIPTION

The following description and drawings are illustrative and are not tobe construed as limiting. Numerous specific details are described toprovide a thorough understanding. However, in certain instances, wellknown or conventional details are not described in order to avoidobscuring the description.

FIG. 1 illustrates an electromagnetic radiation sensing apparatus 100configured to measure a distribution of electromagnetic radiation (suchas infrared radiation) according to at least one embodiment. In FIG. 1 ,the apparatus 100 is configured on a printed circuit board (PCB) 101(such as a PCB of a mobile device or a separate PCB for a stand-aloneradiation monitoring/imaging device). The apparatus 100 includes amicro-mirror chip 102, an image sensor 104, and a beamsplitter unit 106located between the micro-mirror chip 102 and the image sensor 104. Themicro-mirror chip 102 includes a plurality of light reflecting surfaces108 that are illustrated in detail in FIGS. 2-4 . The image sensor 104includes an imaging surface 110. The beamsplitter unit 106 includesbeamsplitters 112 and 114 arranged in sequence. The beamsplitters 112and 114 are aligned along a horizontal axis (i.e., the y-axis) that isparallel to the micro-mirror chip 102 and the imaging surface 110. Asshown, the beamsplitters 112 and 114 can be located side by side. Thehorizontal axis (i.e., the y-axis) is perpendicular to the vertical axis(i.e., the z-axis) and an axis going into and out of the planeillustrated in FIGS. 1-6 (i.e., the x-axis). The x-axis is not shown inFIGS. 1-6 .

As shown, there is a layer of beamsplitters between the micro-mirrorchip 102 and the image sensor 104. The cross section of each of theshown plurality of beamsplitters 112 and 114 in the plane illustrated inFIG. 1 . Further, each beamsplitter includes a partially-reflectivesurface that is oblique to the imaging surface and the micro-mirror chipand that can extend across more than half the height of thebeamsplitter. As shown, each partially-reflective surface extends acrossthe height of the beamsplitter.

As illustrated, the micro-mirror chip 102 can be mounted directly ontothe beamsplitter unit 106 and the beamsplitter unit 106 can be mounteddirectly onto the image sensor 104. The mounting can be performed bygluing the elements together with optical grade adhesive.

The micro-mirror chip 102 can include a set of micro mirrors formed on asubstrate. Each mirror can be a plate having bi-material legs standingon a frame of the substrate. The reflective surface of each mirror platecan be part of a metal layer to form the plurality of light reflectingsurfaces 108. The substrate layer of the mirror plate absorbs radiation(such as infrared radiation) to raise the temperate of the plate. Theradiation absorption surface can be on the opposite side of the platefrom the reflective surface. The bi-material legs bend according to theplate template to rotate the plate and hence the reflective surface. Therotation angle of the plate represents the temperature and/or theintensity of the absorbed radiation by the plate. Some additionalaspects of some embodiments of the micro-mirror chip 102 are disclosedin U.S. Pat. No. 9,810,581.

The image sensor 104 can be a CMOS (or CCD) based image sensor. Theimage sensor 104 can be connected to or include an integrated signalprocessor such as an integrated ASIC. In some embodiments, a signalprocessor can be connected via the PCB 101 (e.g., see signal processingunit 602 of FIG. 6 ).

As illustrated in FIG. 1 , a lens 116 (such as a lens having a sphericalor an aspherical surface, or in some embodiments a flat surface) can beintegrated with the beamsplitter unit 106. In some embodiments, the lens116 can be integrated by cutting the right-side half of the lens 116 offto have a flat surface to attach the lens to the flat surface of thebeamsplitter unit 106. In some other embodiments, the lens 116 is formeddirectly on the beamsplitter unit 106.

The apparatus 100 can have a point light source 126 located at a focalpoint of a lens 116. The lens 116 is shown configured to convert thenon-collimated light rays, such as light rays 118 a, 118 a′, and 118 a″from the light source 126, into collimated light rays 118 b, 118 b′, and118 b″ entering the beamsplitter unit 106. Specifically, the lens 116collimates and produces light rays 118 b, 118 b′, and 118 b″ that areparallel to the imaging surface 110 and the micro-mirror chip 102. Insome embodiments (not shown here) the light rays 118 b, 118 b′, and 118b″ can also converge towards each other to project a smaller image ofthe mirror plane onto the image plane. For instance, lens 116 collimatesnon-collimated light rays 118 a, 118 a′, and 118 a″ into collimatedlight rays 118 b, 118 b′, and 118 b″ respectively. The non-collimatedlight rays 118 a, 118 a′, and 118 a″ emitted from a light source 126,form a cone shape with the tip of the cone at the light source 126. Whenthe light source 126 is positioned at a focal point of the lens 116, thelens 116 converts the non-collimated light rays 118 a, 118 a′, and 118a″ into the collimated light rays 118 b, 118 b′, and 118 b″ that areparallel light rays entering the beamsplitter 112.

The light source 126 can be implemented using a light emitting diode(LED).

As illustrated in FIG. 1 , the beamsplitting unit 106 can have a frontbeamsplitter 112 and a back beamsplitter 114. The front beamsplitter 112splits the incoming collimated light rays (e.g., 118 b) into light raysreflected towards the portion of micro-mirror chip (102) above the frontbeamsplitter 112 and passing through light rays entering the backbeamsplitter 114. The back beamsplitter 114 further splits the passingthrough light rays coming from the front beamsplitter 112 into lightrays reflected towards the portion of micro-mirror chip (102) above theback beamsplitter 114 and light rays passing through the backbeamsplitter 114.

For example, the beamsplitter 112 splits the collimated light ray 118 binto two light rays 118 c and 118 e. The light ray 118 c is reflected bythe beamsplitter 112 towards the portion of the plurality of lightreflecting surfaces 108 of the micro-mirror chip 102 that is positionedright above the beamsplitter, and the light ray 118 e penetrates throughthe partially-reflective surface of the beamsplitter 112 as the incominglight ray for the beamsplitter 114. The light ray 118 c is shownreflecting off one of the reflecting surfaces of the plurality of lightreflecting surfaces 108 as light ray 118 d, as illustrated in detail inFIGS. 2-4 . The light ray 118 d has a portion that passes through thepartially-reflective surface of beamsplitter 112 towards the imagingsurface 114 of the image sensor 104 as light ray 118 dd.

The light ray 118 e, passing through the beamsplitter 112, is reflectedoff partially the partially-reflective surface of the beamsplitter 114as light ray 118 f. The light ray 118 f is shown as reflected by thebeamsplitter 114 towards the portion of plurality of light reflectingsurfaces 108 of the micro-mirror chip 102 that is right above thebeamsplitter 114. The light ray 118 f is shown being reflected off oneof the reflecting surfaces of the plurality of light reflecting surfaces108 as light ray 118 g. The light ray 118 g has a portion penetratingthrough the partially-reflective surface of beamsplitter 114 towards theimaging surface of the image sensor 104 as light ray 118 gg.

As illustrated in FIG. 1 , a radiation lens 120 can be used to directenvironmental radiation rays 124 a, through a radiation filter 122 (theradiation filter being optional), towards to the radiation absorptionsurfaces of the micro mirrors of the micro-mirror chip 102.

FIG. 2 illustrates some parts of the apparatus 100 illustrated in FIG. 1, and further illustrates the micro-mirror chip 102 of the apparatus100.

As shown in FIG. 2 , the radiation filter 122 (which can be omitted insome embodiments) can filter the directed radiation before the radiationto be measured reaches the micro-mirror chip 102. Specifically, asshown, the radiation filter 122 can filter the directed radiation beforethe radiation is received by radiation absorption surfaces 204 of themicro mirrors 202.

The micro mirrors 202 can be arranged into two halves 206 and 208 of themicro-mirrors chip 102, as shown. Reflective surfaces of themicro-mirrors provide the plurality of light reflecting surfaces 108.Also, as shown, the plurality of light reflecting surfaces 108 are onrespective opposing sides to respective radiation absorption surfaces204.

In general, light emitted from a light emitting source (such as thelight source 126, which can be or include a light emitting diode) iseventually reflected off the plurality of light reflecting surfaces 108of the micro-mirror chip 102 according to the orientations of theplurality of light reflecting surfaces 108. And, the orientations resultfrom the respective amounts of radiation received by each of theradiation absorption surfaces 204 of the micro-mirror chip 102. Theorientations of the light reflecting surfaces 108 effect the angles 320and 326 depicted in FIG. 3 . FIG. 2 illustrates the radiation rays 124 ccausing certain micro mirrors 202 to change in orientation, and thuseffecting the angle in which light rays 118 c and 118 f reflect off eachof the respective surfaces of the plurality of light reflecting surfaces108 as respective reflected light rays 118 d and 118 g.

As shown in FIG. 1 , the lens 120 can be integrated with a housing 128of the radiation filter 122 and the micro-mirror chip 102. Also, FIG. 1shows the housing 128 having a recess and opening 129 in which the lens120 can be integrated with the housing 128. The lens 120 is shown inFIGS. 1 and 2 as configured to direct rays of radiation, such asradiation rays 124 a, to the radiation filter 122 as radiation rays 124b (as shown in FIG. 2 ).

The radiation filter 122 can have different filtering characteristics.For example, the radiation imaging lens 120 can be an infrared lens madeof e.g., Germanium, Silicon, polymer, chalcogenide, glass, and the like.

As shown in FIG. 2 , the radiation imaging lens is arranged in relationwith micro mirrors 202 of the micro-mirror chip 102 to form an image ofthe radiation (e.g., infrared radiation) on a mirror plane 200 of themicro mirrors 202. The radiation image is derived from radiation rays124 c.

The radiation intensity provided by radiation rays 124 c can correspondto light ray displacement on the imaging surface 110 of the image sensor104 (such as light ray displacements 402 and 404 as shown in FIG. 4 ).Light ray displacement, produced by a respective micro mirror of themicro mirrors 202, corresponds to the intensity of a pixel of theradiation image derived from the radiation filter 122 at the location ofthe respective micro mirror.

As shown in FIG. 1 , the plurality of light reflecting surfaces 108faces the imaging surface 110, and as illustrated in FIG. 2 , each lightreflecting surface of the plurality of light reflecting surfaces 108functions as a micro mirror that moves independent of the other mirrorsof the plurality of light reflecting surfaces 108 according to radiationabsorbed by the radiation absorption surfaces 204.

FIG. 3 illustrates some parts of the apparatus 100 illustrated in FIG. 1, and further illustrates the beamsplitter unit 106 of the apparatus.

As illustrated in FIGS. 1 and 3 , the beamsplitters 112 and 114 arearranged in sequence in a direction 130 that is parallel to themicro-mirror chip 102 and the imaging surface 110. For example, thebeamsplitters 112 and 114 are positioned side by side along a horizontalaxis (i.e., the y-axis) that is parallel to the micro-mirror chip andthe imaging surface. Note that the direction 130 is also shown in FIGS.1-6 as a direction of reference for the other aspects of and related tothe apparatus 100.

As shown in FIG. 3 , the beamsplitter unit 106 includes beamsplitter 112including a first partially-reflective surface 302 that is oblique tothe imaging surface 110 and the micro-mirror chip 102. The beamsplitterunit 106 also includes the beamsplitter 114 including a secondpartially-reflective surface 304 that is oblique to the imaging surface110 and the micro-mirror chip 102. As shown, the first and secondreflective surfaces 302 and 304 are aligned in sequence along ahorizontal axis (i.e., the y-axis) that is parallel to the micro-mirrorchip and the imaging surface. For example, the reflective surfaces 302and 304 are positioned side by side along a horizontal axis that isparallel to the micro-mirror chip and the imaging surface, such that thesecond partially-reflective surface 304 is configured to further split aportion of the incoming light rays 118 b that pass through the firstpartially-reflective surface 302 and a portion of the incoming lightrays 118 b is reflected towards the first half 206 of the micro-mirrorsand a portion of the incoming light rays 118 b that penetrates the firstpartially-reflective surface 302 is separately reflected towards thesecond half 208 of the micro-mirrors. Each of the reflective surfaces302 and 304 can extend across more than half the height of itsrespective beamsplitter. As shown, the reflective surface 302 extendsacross the height of the beamsplitter 112 and the reflective surface 304extends across the height of the beamsplitter 114.

Each of the first and second reflective surfaces 302 and 304 can becoated with a material that causes exactly 50:50 splitting of light raysat the surfaces.

In order to achieve a uniform illumination on the image plane, the firstpartially-reflective surface 302 needs to be coated with an optical filmwhich has 25% reflection and 75% transmission. The secondpartially-reflective surface 304 needs to be coated with an optical filmwhich has 50% reflection and 50% transmission. Assuming 100% of lightintensity enters the beamsplitter unit 106 (e.g. as light ray 118 b), atthe surface 302 the reflected ray 118 c will be of 25% intensity, whichreflects back from the micro-mirror 108 (assuming theoretically noreflection loss). Thus, beam 118 d has 25% of original intensity.Passing again through surface 302 the transmitted ray 118 dd will haveonly 75% of its 25% relative intensity, meaning the final intensity ofray 118 dd as falling onto the surface 110 is 18.75% of the originalray. Further continuing with light ray 118 e which has been transmittedthrough surface 302 and which carries 75% of original intensity nowtransmits onto the second partially-reflective surface 304 and arelative 50% gets reflected upwards towards the micro mirrors, beingdefined as light ray 118 f. Light ray 118 f carries 37.5% absoluteintensity and gets reflected off the micro-mirrors, turns into light ray118 g and passes through surface 304. At this point, it is losinganother 50% of its intensity. Thus, light ray 118 gg, as transmittedonto the imaging surface 110 has an intensity of 18.75%. Thus, the rayscaptured by the image plane appear in same or similar intensities withsuch a configuration.

Also, as shown in FIG. 3 , the image sensor includes a first half 306and a second half 308.

As illustrated in FIG. 3 , the first partially-reflective surface 302 isoblique to the imaging surface 110 at an angle 310. The firstpartially-reflective surface 302 is oblique to the micro-mirror chip 102at an angle 312. The second partially-reflective surface 304 is obliqueto the imaging surface 110 at an angle 314. The semi-second reflectivesurface 304 is oblique to the micro-mirror chip 102 at an angle 316.

As described in detail herein, in some embodiments, the angles 310, 312,314, and 316 are 45-degree angles. In some embodiments, when themicro-mirror chip 102 and the imaging surface 110 are not parallel toeach other, the angles 310 and 314 can be different from the angles 312and 316 (not shown in the drawings). Also, in some embodiments, theangles 310, 312, 314, and 316 can be the same but are less than orgreater than 45-degree angles.

As shown in FIG. 3 , the beamsplitter 112 is configured to split a lightray 118 b to a light ray 118 c and a light ray 118 e. As shown, thelight ray 118 c reflects from the beamsplitter 112 at the firstpartially-reflective surface 302 at an angle 318 towards first half 206of the micro-mirror chip 102. The light ray 118 e passes through thebeamsplitter 112 towards the beamsplitter 114. Each light reflectingsurface of the plurality of light reflecting surfaces 108 of the firsthalf 206 of the micro-mirror chip 102 reflects a light ray 118 d, at anangle 320, which is split at the first reflective surface 302 to a lightray 118 dd and another light ray (not shown in the drawings) where thelight ray 118 dd passes through the first partially-reflective surface302 to reach the imaging surface 110 at the first half 306 of the imagesensor 104, at an angle 322.

Also, as shown in FIG. 3 , the light ray 118 e passes through thebeamsplitter 112 and is split by the beamsplitter 114 into a light ray118 f and another light ray (not shown in the drawings), where the lightray 118 f is reflected by the beamsplitter 114 at a secondpartially-reflective surface 304 at an angle 324 towards second half 208of the micro-mirror chip 102 and the other light ray passes through thebeamsplitter 114. Each light reflecting surface of the plurality oflight reflecting surfaces 108 of the second half 208 of the micro-mirrorchip 102 reflects a light ray 118 g, at a respective angle 326indicative of its exposure to radiation 124 c. The reflected light ray119 g is split at the second reflective surface 304 to a light ray 118gg and another light ray (not shown in the drawings) where the light ray118 gg passes through the second reflective surface 304 to the imagingsurface 110 at the second half 308 of the image sensor 104, at an angle328.

In some embodiments, the beamsplitter unit includes a first beamsplitterhaving a first 45-degree partially-reflective surface that is 45 degreesfrom the imaging surface and 45 degrees from the micro-mirror chip. Thefirst 45-degree partially-reflective surface can extend across more thanhalf the height of the first beamsplitter (e.g., the first 45-degreepartially-reflective surface can extend across the entire height of thefirst beamsplitter). Also, in such embodiments, the beamsplitter unitcan include a second beamsplitter including a second 45-degreepartially-reflective surface that is 45 degrees from the imaging surfaceand 45 degrees from the micro-mirror chip. The second 45-degreepartially-reflective surface can extend across more than half the heightof the second beamsplitter (e.g., the second 45-degreepartially-reflective surface can extend across the entire height of thesecond beamsplitter).

As mentioned herein, each of angles 310, 312, 314, and 316 can be 45degrees. This can occur when the beamsplitters are align in sequence andparallel to the micro-mirror chip and the imaging surface of the imagesensor. For example, this can occur when the beamsplitters are alignedside by side along a horizontal axis that is parallel to themicro-mirror chip and the imaging surface of the image sensor. Althoughit appears in FIG. 3 that all the angles 310, 312, 314, and 316 areapproximately 45 degrees, for the present disclosure it shall beunderstood that the aforesaid angles can vary.

In such embodiments with the beamsplitters having 45-degree reflectivesurfaces and the beamsplitters being aligned in sequence and parallel tothe micro-mirror chip and the imaging surface, the angle 318 is a90-degree angle with respect to the direction 130 (the direction 130being parallel to the micro-mirror chip 102 and the imaging surface110). To put it another way, in such embodiments, the angle 318 is a90-degree angle with respect to the micro-mirror chip 102 and theimaging surface 110. In such embodiments, when a light reflectingsurface of the plurality of light reflecting surfaces 108 that reflectsthe light ray 118 c is aligned parallel to the direction 130, the angle320 is a zero-degree angle in that the light ray 118 d is reflecteddirectly back along the path of the light ray 118 c. Also, in suchembodiments, when the light reflecting surface that reflects the lightray 118 c as ray 118 d is aligned parallel to the direction 130, theangle 322 is a 90-degree angle with respect to the direction 130.

Likewise, in such embodiments with the beamsplitters including 45-degreereflective surfaces and being aligned in sequence and parallel to themicro-mirror chip and the imaging surface, the angle 324 is a 90-degreeangle with respect to the direction 130 (the direction 130 beingparallel to the micro-mirror chip 102 and the imaging surface 110). Toput it another way, in such embodiments, the angle 324 is a 90-degreeangle with respect to the micro-mirror chip 102 and the imaging surface110. In such embodiments, when a light reflecting surface of theplurality of light reflecting surfaces 108 that reflects the light ray118 f is parallel to the direction 130, the angle 326 is a zero-degreeangle in that the light ray 118 g is reflected directly back along thepath of the light ray 118 f. Also, in such embodiments, when the lightreflecting surface that reflects the light ray 118 f is parallel to thedirection 130, the angle 328 is a 90-degree angle with respect to thedirection 130.

Also, in such embodiments with the beamsplitters including 45-degreereflective surfaces and being aligned in sequence and parallel to themicro-mirror chip and the imaging surface, a light ray of 100% intensity(e.g., the light ray 118 b) can be split by a first beamsplitter (e.g.,the beamsplitter 112) to two light rays each of 50% intensity (e.g., thelight rays 118 c and 118 e). One of the two split light rays of 50%intensity can reflect from the first beamsplitter towards a first half(e.g., first half 206) of the micro-mirror chip and the other splitlight ray of 50% intensity passes through the first beamsplitter towardsthe second beamsplitter (e.g., the beamsplitter 114).

Also, in such embodiments with the beamsplitters including 45-degreereflective surfaces and being aligned in sequence and parallel to themicro-mirror chip and the imaging surface, each light reflecting surfaceof plurality of light reflecting surfaces (e.g., the plurality of lightreflecting surfaces 108) of the first half of the micro-mirror chipreflects a light ray (e.g., light ray 118 d) that is split at a first45-degree reflective surface (e.g., the first reflective surface 302)such that the split ray passes through the first 45-degree reflectivesurface to a first half (e.g., first half 306) of the imaging surface at25% intensity (e.g., light ray 118 dd can have an intensity of 25% insuch embodiments). The light ray reflecting off the first reflectivesurface of the first beamsplitter at this point is not shown in thedrawings.

Also, in such embodiments with the beamsplitters including 45-degreereflective surfaces and being aligned in sequence and parallel to themicro-mirror chip and the imaging surface, the split light ray passingthrough the first beamsplitter of 50% intensity (e.g., light ray 118 e)is split by the second beamsplitter to two light rays each of 25%intensity. The light ray passing through the second beamsplitter at thispoint is not shown in the drawings. One of the two split light rays of25% intensity (e.g., light ray 118 f) reflects from the secondbeamsplitter towards a second half (e.g., second half 208) of themicro-mirror chip, and the other split light ray of 25% intensity (notshown in the drawings) passes through the second beamsplitter.

Also, in such embodiments with the beamsplitters including 45-degreereflective surfaces and being aligned in sequence and parallel to themicro-mirror chip and the imaging surface, each light reflecting surfaceof plurality of light reflecting surfaces of the second half of themicro-mirror chip reflects a light ray (e.g., light ray 118 g) that issplit at a second 45-degree reflective surface (e.g., second reflectivesurface 304) such that the split ray passes through the second 45-degreereflective surface to a second half (e.g., second half 308) of theimaging surface at 12.5% intensity (e.g., light ray 118 gg can have anintensity of 12.5% in such embodiments). The light ray reflecting offthe second reflective surface of the second beamsplitter at this pointis not shown in the drawings.

FIG. 4 illustrates some parts of the apparatus 100 illustrated in FIG. 1, and further illustrates mechanisms for displacements (such asdisplacements 402 and 404) of reflected light rays on the imagingsurface 110 of the image sensor 104 of the apparatus 100 to determinethe intensity of electromagnetic radiation at corresponding locations ofmicro mirrors 202 of the micro-mirror chip 102 of the apparatus 100.FIG. 4 also shows the micro-mirror chip 102 in the same level of detailas FIG. 2 , and depicts the integration of the micro-mirror chip 102with the beamsplitter unit 106 and the image sensor 104.

Regarding the mechanisms for displacements, FIG. 4 shows dotted arrows118 d″, 118 dd″, 118 g″, and 118 gg″ that represent the positions of thecorresponding light rays reflected by respective micro mirrors 202 in aninitial dotted line position of the respective micro mirrors 202. Afterthe micro mirrors 202 rotate from the dotted line position to the solidline position (as a consequence of absorbing or sensing sufficientradiation to rotate the mirrors), the light rays of the rotated micromirrors 202 move from the initial location to a subsequent location asillustrated by the solid arrows 118 d′, 118 dd′, 118 g′, and 118 gg′.

The measurements of the light ray displacements 402 and 404 can be usedto compute an angle of rotation of the corresponding micro mirrors 202.The rotation of a respective one of micro mirrors 202 is proportionatelya function of the radiation intensity on the respective one of theradiation absorption surfaces 204 of a respective micro mirror; thus,the measured displacements 402 and 404 can be used to calculate theradiation intensity on the radiation absorption surfaces 204 of themicro mirrors 202.

The measurement of the light ray displacement (e.g., displacements 402or 404) can be performed for each one of micro mirrors 202 and used todetermine the distribution of the radiation intensity on a single micromirror or on an array of the micro mirrors.

In one embodiment, a photodetector is used to capture the image formedon the imaging surface 110 of the image sensor 104, identify individuallight spots derived from corresponding light rays and corresponding torespective micro mirrors 202, determine the locations of the lightspots, and compute displacements of the respective light spotscorresponding to the displacements of the light rays (such asdisplacements 402 and 404); and thus, compute the light intensityassociated with the radiation intensity on the micro mirrors 202.

As shown in FIGS. 2 and 4 , the y-axis is in the direction of the row ofmicro mirrors 202 and is parallel to the imaging surface 110 as well asthe direction 130 described herein. The light ray displacements 402 and404, and hence the corresponding light spot displacements on the imagingsurface 110, are along the y-axis direction. The mirror plane 200 andthe imaging surface 110 are separated by a distance 406 along the z-axisthat is perpendicular to the mirror plane 200 and the y-axis direction.As illustrated, the height of the beamsplitter unit 106 can be thedistance 406.

The distance 406 along the direction perpendicular of the row of mirrors(i.e., the z-axis) can include the beamsplitter unit 106 (as shown inFIGS. 1-6 ). Thus, to prevent the beamsplitter unit 106 from interferingwith the reflected light from the micro-mirror chip, in someembodiments, the reflected light can travel in a path that avoids thebeamsplitter unit 106.

Not shown in the drawings, in some embodiments, light rays can bereflected from one of the plurality of light reflecting surfaces 108 atan angle from the mirror plane 200 in a direction along the x-axis inthe x-z plane. The x-z plane is perpendicular to the y-z plane of FIGS.1-6 . Thus, in such embodiments, light rays generally travel along thedirection of the row of micro mirrors (such as shown by micro mirrors202) onto the mirror plane 200 in the y-z plane; and, after beingreflected by the micro mirrors, the rays travel along the same directiononto the imaging surface of the image sensor but skewed in the x-z planeor in the direction of the x-axis which goes into or out of the y-zplane of FIGS. 1-6 . In one embodiment, there are no structural and/oroptical components on the light path between the micro mirrors and theimaging surface. In these ways for example, the reflected light cantravel in a path that avoids the beamsplitter unit 106 when travelingtowards the imaging surface 110.

As shown in FIG. 4 , the imaging surface 110 is in parallel with themirror plane 200. Thus, when the micro mirrors 202 are in the initialpositions that are aligned with the mirror plane 200, the lightreflected by different micro mirrors 202 travels equal distances fromrespective light reflecting areas of the plurality of light reflectingsurfaces 108 of the micro mirrors to the imaging surface 110. As aresult, equal rotations of the micro mirrors 202, due to equal radiationintensity applied on the radiation absorption surfaces 204 of the micromirrors 202, result in equal light ray displacement on the imagingsurface 110. This arrangement can simplify the calibration for computingthe light intensity from the light ray displacement and/or improveaccuracy and/or ensure uniform signal generation and uniform sensitivityin the conversion from radiation intensity to light ray displacement.

FIG. 4 illustrates the measuring of displacements (e.g., displacements402 and 404) of reflected light rays on the imaging surface 110 todetermine the intensity of electromagnetic radiation at the location ofmicro mirrors according to one embodiment.

FIGS. 2 and 4 illustrate a single row of mirrors. However, themicro-mirror chip 102 can have multiple rows of mirrors, which cannot beshown by the two dimensions of FIGS. 1-6 .

Not shown in the drawings, in some embodiments, each one of the micromirrors 202 on its respective light reflecting surface of the pluralityof light reflecting surfaces 108 has a light reflecting area and anon-reflective area. The shape and size of the light reflecting area ofeach micro mirror defines a light spot reflected by the micro mirror onto the imaging surface. In some embodiments, micro mirrors of a chiphave the same shape and size in their light reflecting areas.Alternatively, different micro mirrors in a chip can have differentshapes and/or sizes in their light reflecting areas, resulting indifferently shaped reflected light spots on the imaging surfaces.

The different optical characteristics of the light reflecting areas canbe used to improve the accuracy in correlating the light spots on theimaging surface with the corresponding micro mirrors responsible forreflecting the light spots. Different optical characteristics can beachieved by using varying the shape, size, reflection rate, orientation,and/or polarization, etc. of in the reflecting surfaces of the pluralityof light reflecting surfaces 108. Further, symbols or graphical patternscan be applied (e.g., etched or overlaid) on the light reflecting areasto mark the micro mirrors such that the micro mirrors responsible forgenerating the light spots on the imaging surface can be identified fromthe shape, size, orientation, polarization, intensity and/or markers ofthe corresponding light spots captured on the imaging surface.

FIG. 5 illustrates another apparatus 500 configured to measure adistribution of electromagnetic radiation according to at least oneother embodiment where a light source 502 for the apparatus 500 is partof the PCB 101.

As shown, the apparatus 500 includes or interacts with many elementsthat are similar to elements of or that interact with the apparatus 100of FIG. 1 . Different from apparatus 100, apparatus 500 includes ahousing 504 and the light source 502 (which can be or include a lightemitting diode) that is part of the PCB 101 and perpendicular to a largeplane of the PCB.

The light source 502 emits light rays including light ray 506 upwards inthe general direction of the z-axis. To reflect light towards the lens116 integrated with the beamsplitter unit 106, the housing 504 includesan angled wall 508 that is skewed from the z-axis at angle 510. In someembodiments, the angle 510 is 45 degrees so that a center ray, e.g.,light ray 506, of the light rays emitted by the light source 502, isreflected at a 90-degree angle towards the lens 116. At least a portionof the angled wall 508 is reflective and functions as a mirror such thata mirror image of the light source 502 is at a focal point of the lens116. Thus, the lens 116 can convert the non-collimated light raysreflected by the wall 508 into collimated light rays 118 b in a way asillustrated in FIG. 1 . The reflective wall 508 allows the light source502 to be configured on the PCB 101 and/or reduce the length of theapparatus 500 along the y-axis.

Also, as shown, the apparatus 500 includes a recess and opening 512 inwhich the lens 120 can be integrated with the housing 504. Also, similarto the apparatus 100 of FIG. 1 , the lens 120 can be integrated with thehousing 128 of the radiation filter 122 and the micro-mirror chip 102.Also, FIG. 5 shows the housing 128 having a recess and opening, which issimilar to the recess and opening 129, in which the lens 120 can beintegrated with the housing 128.

The additional features of apparatus 500 and some of their alternativesare further described in related Prov. U.S. Pat. App. Ser. No.62/791,193 originally titled “ON-BOARD RADIATION SENSING APPARATUS”. Inthe related application originally titled “ON-BOARD RADIATION SENSINGAPPARATUS”, at least some embodiments disclosed relate to on-boardelectromagnetic radiation detection using beam splitting in general andmore particularly but not limited to the on-board sensing of infrared(IR) radiation using beam splitting in a radiation sensing apparatus.The entire disclosure of the related application originally titled“ON-BOARD RADIATION SENSING APPARATUS” is hereby incorporated herein byreference.

FIG. 6 illustrates another apparatus 600 configured to measure adistribution of electromagnetic radiation according to at least oneother embodiment where a signal processing unit 602 is connected to theimage sensor 104 via the PCB 101.

As shown, the apparatus 600 includes or interacts with many elementsthat are similar to elements of or that interact with the apparatus 100of FIG. 1 . Different from apparatus 100, apparatus 600 includes thesignal processing unit 602. Alternatively, a signal processing unit canbe indirectly connected to the image sensor 104, such as a remote signalprocessing unit connected to the image sensor through a computer networkor through a connector. In other embodiments, a signal processing unitcan be directly connected to the PCB 101, and in such examples acoupling on the PCB 101 can connect the image sensor 104 and the signalprocessing unit (as illustrated in FIG. 6 ).

In some embodiments, a signal transmitting unit is coupled with thesignal processing unit 602 or its alternative to transmit the image datacaptured by the image sensor 104 and/or the measuring data processed bythe signal processing unit 602 or its alternative. The image datacaptured by the image sensor 104 and/or the measuring data processed bythe signal processing unit 602 or its alternative indicate the light raydisplacements (such as displacements 402 and 404), the micro mirrorrotations, and the intensity of the radiation (such as the intensity ofradiation rays 124 c).

The signal processing unit 602 can be programmed for customizedprocessing of designated applications. The signal processing unit 602can process the reflected light ray displacements (such as displacements402 and 404) and generate corresponding electrical signal gains. Thesignal can be further processed and for example displayed to the enduser via an external display. In one example, signals processed by thesignal processing unit 602 are transmitted through a communication portwirelessly to a portable device, where the end user can see thegenerated signals and has the ability to control or interact through auser interface with the apparatus 600 or the signal processing unit 602.The signals can be transmitted and exchanged through any wired orwireless transmission method, using e.g. a USB, Bluetooth, Wi-Fi, etc.The end user's display and interface can include any device, for examplea smartphone, tablet, laptop computer, etc.

FIGS. 7A, 7B, and 7C illustrate a construction for the structure of abeamsplitter unit according to at least one embodiment (such as thebeamsplitter unit 106 of FIGS. 1-6 ). A beneficial feature of thebeamsplitter unit described herein is that its height is reduced so thatthe apparatus using the beamsplitter unit has a reduced height as well.This is especially useful for applications with mobile devices. Thereduced height of the beamsplitter unit and the apparatus housing thebeamsplitter unit can allow for including the beamsplitter unit and theapparatus in a mobile device. The mobile device can be any electronicdevice small enough to be held and operated by one or two hands of aperson. For instance, the beamsplitter unit described herein can have aheight of less than 4 millimeters, which allows for use of thebeamsplitter unit within many different types of mobile devices.

The beamsplitter unit described herein is similar to a horizontalarrangement of half-sliced parts of one beamsplitter. To put it anotherway, the beamsplitter unit is similar to a device including half-slicedparts of one beamsplitter that have been rearranged so that the twohalf-sliced parts are merged sequentially in a horizontal manner.

FIG. 7A illustrates one beamsplitter 702. Beamsplitter 702 of FIG. 7Aincludes one reflective surface 704 (e.g., one 45-degree reflectivesurface). FIG. 7B shows slices 706 and 708 of the beamsplitter 702. Theslices 706 and 708 divide the beamsplitter in to four parts. Two of theparts can include the beamsplitters 112 and 114. And, as shown in FIGS.7B and 7C, the beamsplitters can include the reflective surfaces 302 and304 of FIG. 3 respectively. FIG. 7C shows the merged beamsplitters 112and 114, which are merged into structure 710. As shown, there is an area712 in front of the beamsplitter 112, an area 714 between thebeamsplitter 112 and the beamsplitter 114, and an area 716 behind thebeamsplitter 114.

The respective lengths 718 and 720 of each of the beamsplitters 112 and114 further clarify the boundaries of the areas 712, 714, and 716.

The structure 710 of FIG. 7C can be formed by initially cutting thebeamsplitter 702 according to slices 706 and 708 of FIG. 7B, and thenattaching the two parts of beamsplitter 702 that have the reflectivesurfaces 302 and 304. Also, the structure 710 of FIG. 7C can be formedby adding three blocks of transparent materials. The left-side block ofthe three blocks can include an integrated lens such as a lens similarto lens 116 of FIG. 1 . The integrated lens is not shown in FIG. 7C. Thethree blocks can provide the areas 712, 714, and 716. Along with theleft-side block (which can include an integrated lens), a second spacerblock can be attached between the beamsplitters 112 and 114, and a thirdspacer block can be attached to the right of the beamsplitter 114 toderive the structure 710 of FIG. 7C.

It can be beneficial to include the spaces provided by areas 712, 714,and 716 in the beamsplitter unit. The spaces are beneficial in that theycan reduce the effect inferring light rays that occur from the differentlight refractions that occur in the beamsplitter unit 106. Suchinferring light rays interfere with the light rays used to detectradiation when there is not sufficient space between the reflectingsurfaces of the beamsplitters 112 and 114. Thus, the areas 712, 714, and716 can provide the sufficient space to reduce interference.

Another way to reduce interference is to increase the length of eachbeamsplitter of the beamsplitter unit 106. Since it is desirable toinclude the disclosed apparatuses in mobile devices, increasing theheight of beamsplitters to reduce interference is not a practical optionconsidering that many mobile devices have a thin form.

To reduce interference for apparatuses to be used within mobile devices,in some embodiments, the distance between the micro-mirror chip and theimaging sensor is equal to or less than either of the lengths of theimaging sensor and the micro-mirror chip. For example, the distancebetween the micro-mirror chip and the imaging sensor is half or lessthan half of the length of the imaging sensor. Also, the distancebetween the micro-mirror chip and the imaging sensor can be half or lessthan half of the length of the micro-mirror chip.

In some embodiments, the length of each beamsplitter of the beamsplitterunit 106 (such as each respective length 718 and 720 of beamsplitters112 and 114) is at most half of the length of the imaging sensor or themicro-mirror chip used with the beamsplitter unit. In some embodimentsincluding such embodiments where the length of each beamsplitter of thebeamsplitter unit is at most half of the length of the imaging sensor orthe micro-mirror chip used with the beamsplitter unit, the micro-mirrorchip and the image sensor can be the same length.

Also, the length of the beamsplitter unit can be greater than, lessthan, or equal to the lengths of the imaging sensor and the micro-mirrorchip. For example, the length of the image sensor and the total lengthof the beamsplitter unit can be the same or the beamsplitter unit canhave a greater or lesser length than the image sensor. Also, the lengthof the micro-mirror chip and the total length of the beamsplitter unitcan be the same or the beamsplitter unit can have a greater or lesserlength than the micro-mirror chip.

In some embodiments, the sequence of beamsplitters can include more thantwo beamsplitters. For example, the sequence can have three or four oreven more beam splitting planes.

Also, described herein is a printed circuit board arrangement with aflexible part for an electromagnetic radiation detector. In someembodiments, the electromagnetic radiation sensing using beam splittingin a radiation sensing apparatus includes the light source mounted on aflexible part (e.g., see the flexible part 802 depicted in FIG. 8 ). Inother words, variations of the apparatuses described herein can includea light source mounted to a flexible part (e.g., see the flexible part802 depicted in FIG. 8 ). In some embodiments, a light source, such as alight-emitting diode (LED), is attached to a flexible part of theradiation sensing apparatus or a flexible part of the PCB (printedcircuit board) in which the apparatus is attached (e.g., see theflexible part 802 depicted in FIG. 8 ). By bending the flexible part, abeam emitted from the light source can be directed towards abeamsplitter (e.g., see FIG. 8 ).

With using the flexible part, the light ray from the light source (suchas from an LED) can hit the beamsplitter perpendicularly. Also, thelight source can be fixed to the PCB. In some embodiments, the lightsource can be fixed to a flexible part of the PCB or a flexible part ofthe apparatus that is attached to the PCB. Also, in some embodiments(not depicted), the apparatus can include a reflective wall (e.g.,angled wall 508 depicted in FIG. 5 ) and a flexible part (e.g., see theflexible part 802 depicted in FIG. 8 ), such that the flexible part isflexed so that a beam is emitted towards the reflective surface from alight source mounted to the flexible part, and then reflected towardsthe beamsplitter from the reflective surface.

In some embodiments, the light source is attached to a flexible part ofthe PCB and the flexible part is bent upwards. In such example, theflexible part can be attached on a vertical side (front) wall of thehousing of the apparatus. The wall can have a small opening (such as apinhole) for the light source to emit beams through the opening.

In some embodiments, the flexible part (flex PCB) is part of the PCB. Astiffener and a low-profile board-to-board connector can connect theflex PCB to a main PCB in the assembly. Also, a lens and the outer wallsof the apparatus can include a molded plastic shell glued on the PCB.The lens can be the lens 120 as shown in FIG. 8 .

In the foregoing specification, embodiments of the disclosure have beendescribed with reference to specific example embodiments thereof. Itwill be evident that various modifications can be made thereto withoutdeparting from the broader spirit and scope of embodiments of thedisclosure as set forth in the following claims. The specification anddrawings are, accordingly, to be regarded in an illustrative senserather than a restrictive sense.

What is claimed is:
 1. An apparatus comprising: a plurality of mirrors;a sensor having a surface to receive light rays reflected from themirrors; and at least one beamsplitter located between the mirrors andthe sensor, wherein the beamsplitter comprises at least onepartially-reflective surface configured to partially reflect light raystowards the mirrors.
 2. The apparatus of claim 1, wherein the at leastone partially-reflective surface includes first and secondpartially-reflective surfaces aligned in sequence.
 3. The apparatus ofclaim 2, wherein a first beamsplitter comprises the firstpartially-reflective surface, and a second beamsplitter comprises thesecond partially-reflective surface.
 4. The apparatus of claim 3,wherein the first and second beamsplitters are positioned side by side.5. The apparatus of claim 1, wherein each mirror comprises a lightreflecting surface on one side of the mirror, and a radiation absorptionsurface on an opposing side of the mirror.
 6. The apparatus of claim 1,wherein each mirror is configured to have an orientation determined byan amount of radiation received by a radiation absorption surface of themirror.
 7. The apparatus of claim 6, wherein light is reflected off alight reflecting surface of each mirror according to the respectiveorientation of the mirror.
 8. The apparatus of claim 1, wherein thelight rays reflected from the mirrors form an image on the surface ofthe sensor.
 9. The apparatus of claim 8, wherein displacements of thelight rays reflected from the mirrors on the surface of the sensordetermine an intensity of radiation at corresponding locations of themirrors.
 10. The apparatus of claim 9, further comprising a signalprocessing unit configured to: identify light spots derived fromcorresponding light rays and corresponding to respective mirrors;determine locations of the light spots; and compute displacements of therespective light spots corresponding to the displacements of the lightrays.
 11. The apparatus of claim 8, wherein the sensor is aphotodetector configured to capture the image.
 12. The apparatus ofclaim 8, further comprising a signal processing unit connected to thesensor, wherein the signal processing unit is configured to determine adistribution of radiation intensity based on displacements of the lightrays reflected from the mirrors on the surface of the sensor.
 13. Anapparatus comprising: a substrate; a plurality of mirrors formed on thesubstrate, wherein each mirror comprises a light reflecting surface onone side of the mirror, and a radiation absorption surface on anopposing side of the mirror; a sensor having a surface to receive lightrays reflected from the mirrors; a first beamsplitter comprising a firstpartially-reflective surface configured to partially reflect light raystowards the mirrors; and a second beamsplitter comprising a secondpartially-reflective surface configured to partially reflect light raystowards the mirrors; wherein the first and second beamsplitters arepositioned between the substrate and the sensor.
 14. The apparatus ofclaim 13, wherein the first and second partially-reflective surfaces arealigned in sequence.
 15. The apparatus of claim 13, wherein each of thefirst and second partially-reflective surfaces is oblique to the surfaceof the sensor.
 16. The apparatus of claim 13, wherein each mirror isconfigured to have an orientation determined by an amount of radiationreceived by the radiation absorption surface of the mirror.
 17. Theapparatus of claim 16, wherein light is reflected off the lightreflecting surface of each mirror towards the surface of the sensoraccording to the respective orientation of the mirror.
 18. An apparatuscomprising: a plurality of mirrors; a sensor having a surface to receivelight rays reflected from the mirrors; a first beamsplitter comprising afirst partially-reflective surface configured to partially reflect lightrays towards the mirrors; and a second beamsplitter comprising a secondpartially-reflective surface configured to partially reflect light raystowards the mirrors; wherein the first and second partially-reflectivesurfaces are aligned in sequence.
 19. The apparatus of claim 18,wherein: displacements on the surface of the sensor of the light raysreflected from the mirrors determine an intensity of radiation atcorresponding locations of the mirrors; and a respective displacementfor each mirror is associated with an amount of radiation received by aradiation absorption surface of the respective mirror.
 20. The apparatusof claim 18, wherein a light ray coming from a light source is split bythe first beamsplitter to two light rays, and wherein one of the twolight rays reflects from the first beamsplitter towards the mirrors andthe other of the two light rays passes through the first beamsplittertowards the second beamsplitter.